This invention relates to a power supply system for driving a vehicle using a plurality of capacitors in an electric power storage device.
An electric power storage device, as a power source, mounted on a vehicle equipped with an electric motor, discharges when the vehicle accelerates or runs at constant speed, and charges when the vehicle is braked. One known type of such an electric power storage device uses a plurality of capacitors, for example, electric double layer capacitor cells. Japanese Laid-Open Patent No. Hei-8-168182 published in 1996 discloses a power supply system using capacitors.
FIG. 7 shows a schematic diagram of a prior art of a power supply system using a capacitor electric power storage device mounted on a series type hybrid vehicle. An engine 1 drives a generator 2. Power generated by the generator 2 is supplied to an electric power storage device 4 through a rectifier 3, and simultaneously supplied to a motor 6 through an inverter 5. Wheels, not shown, are driven by the motor 6. In the drawing, 8 is an auxiliary electric power storage device comprised of chemical batteries, and 7 is a DC-DC converter charging the auxiliary electric power storage device 8, and 9 is auxiliary machine. The electric power storage device 4 is a system in which a plurality of capacitors, 41, 42, 43, . . . are connected in series, and the conventional chemical secondary battery is replaced with these capacitors. When the vehicle is accelerated or is running at constant speed, a part or all of the power generated by the generator 2 is charged in the electric power storage device 4, and the power generated by the generator 2 and the power in the electric power storage device 4 are supplied to the motor 6 through the inverter 5. When braking the vehicle, the power generated in the motor 6 is regenerated to the electric power storage device 4 through the inverter 5.
Furthermore, because there are variations in electrostatic capacity among respective capacitors 41, 42, 43 . . . , some capacitors, 41, 42, 43, . . . may be overcharged, over-discharged or inversely charged when the electric power storage device 4 is repeatedly charged and discharged. This causes a problem in that the capacitors are deteriorated. If, so as to avoid this problem, a range of the terminal voltage of the capacitors 41, 42, 43 . . . is limited, electrostatic energy proportional to the square of the terminal voltage is remarkably reduced.
This invention provides a power supply system for a vehicle, the power supply system comprising: an electric power storage device including a parallel connection of a plurality of capacitor banks, each of the capacitor banks having a plurality of capacitors connected in series; a bank halting means for halting at least one of the plurality of capacitor banks depending on the current flowing through the electric power storage device; individual discharging circuits each individually discharging the electric charge of the respective capacitors; and a discharging control means for equalizing the terminal voltage of the capacitors in the halted capacitor bank by individually discharging the electric charge of the respective capacitors.
Even if there are some variations in electrostatic capacity among respective capacitors, this invention allows storage electric power to be secured at a maximum by reducing variations in electric potential among respective capacitors. Further, even when the electric power storage device is repeatedly charged and discharged, this invention allows variations in electric potential among respective capacitors to be reduced in a driving state in which at least one of the capacitor banks can be halted. This prevents a part of the respective capacitors from being overcharged and over-discharged, or inversely charged. Thus, efficiency in the electric power storage device can be enhanced, and a long-life power supply system for a vehicle can be realized.
In this invention, the capacitor bank may comprise: a capacitor series circuit having a plurality of capacitors connected in series, and being connected to the power circuit in parallel; and a capacitor parallel circuit in which the capacitors arranged in the same series stage of the capacitor series circuit are connected in parallel. The individual discharging circuit can equalize the respective terminal voltages by individually discharging the electric charge in the respective capacitor parallel circuits.
Thus, even if there are some variations in electrostatic capacity among the respective capacitor parallel circuits, this invention allows the storage electric power to be secured at a maximum by reducing variations in electric potential among the respective capacitor parallel circuits.
Further, since this invention provides an individual voltage detecting circuit for detecting the terminal voltage of the respective capacitor parallel circuits, the bank halting means can be constituted so as to halt the capacitor bank with a large variation in the detected terminal voltage of the capacitor parallel circuits. Thus, the terminal voltage of respective capacitor parallel circuits is detected, and then variations in electric potential among the respective capacitors can be efficiently reduced by halting the capacitor bank which has a large variation in the detected terminal voltage of the capacitor parallel circuits.
Since this invention provides an individual voltage detecting circuit for detecting the terminal voltage of the respective capacitor parallel circuits, the discharging control means can be arranged so as to allow the respective capacitors to discharge to the average terminal voltage of the capacitor parallel circuits. The variations in electric potential among the respective capacitors can be efficiently reduced by detecting the terminal voltage of the respective capacitor parallel circuits and allowing the respective capacitors to discharge to the average terminal voltage of the capacitor parallel circuits.
Since this invention provides a discharging state detecting means for detecting a discharging state of the capacitor discharged by the individual discharging circuit, variations in electric potential among the respective capacitors can be efficiently reduced by detecting the discharging state of the capacitor discharged by the individual discharging circuit.